The structure, organization, expression, and evolution of the genes for the crystalline of the eye lens have been examined. Sequencing of cDNAs for mouse AlphaA- and Beta-crystallin and chicken Delta-crystallin polypeptides have provided primary structures for these lens proteins. The exon-intron structure of a mouse Beta-crystallin gene was shown to relate to the folding units predicted for its protein and differed greatly from that of the highly interrupted Delta-crystallin genes in chickens and ducks. Analysis of cDNAs encoding four mouse Gamma-crystallin and four chicken Beta-crystallin polypeptides revealed that these proteins have related gene families which probably arose from a common ancestral Beta/Gamma-gene that internally duplicated. A specific deficiency of a 27K Beta-crystallin mRNA was found in the Philly mouse cataract, and a selective loss of Delta-crystallin mRNA was shown in the chicken lenses three to five months after hatching. Evidence has been obtained for a new crystallin in the turtle lens. Together, the data extend our knowledge of the molecular genetics of the crystallins.